This invention relates generally to a system and method for detecting gas leakage from an enclosed space, such as fuel vapor leakage from an evaporative emission space of a motor vehicle fuel system, especially to a system and method where a pump, such as a diaphragm pump, creates superatmospheric pressure in the space during a test.
A known on-board evaporative emission control system for a motor vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of a fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, the purge valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine""s combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain motor vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space.
One known type of vapor leak detection system for determining integrity of an evaporative emission space performs a leak detection test by positively pressurizing the evaporative emission space using a positive displacement diaphragm pump. The diaphragm is reciprocated to create test pressure. Commonly owned U.S. Pat. No. 6,192,743, issued Feb. 27, 2001, discloses a module comprising such a pump.
Known test methods include creating superatmospheric pressure in the closed space being tested and detecting changes that are indicative of leakage. One method comprises measuring a characteristic of pump operation. An example of a time-based measurement is a measurement of how frequently a diaphragm pump must be cycled in order to maintain pressure. Other methods of measurement are pressure-based, such as measuring the rate at which pressure decays. Those methods can provide accuracy when ambient conditions are relatively stable, such as when a vehicle has been parked for an extended period of time. Less stable conditions may impair accuracy of measurements. The dynamics of operating a vehicle may prevent a leak test method from providing consistently accurate results. For example, movement of liquid fuel in a tank, i.e. fuel slosh, might create certain pressure anomalies that could give a false result for a leak test.
The inclusion of various filters, both electrical and mechanical, may mitigate the effects of such anomalies. Even with the presence of such aids, it is believed that further improvement toward assuring consistent accuracy of test results is desirable, and it is toward that objective that the present invention is directed.
Commonly owned pending U.S. patent application Ser. No. 09/896,247, filed Jun. 29, 2001, discloses a system and method that compensates for changes in the output efficiency of a pump due to factors such as temperature, age, friction, etc., so that a leak test can be performed and completed within a specified window of time as the pump efficiency changes. The pump is operated in a manner that creates a succession of pressurizing pulse bursts. Each burst contains a number of pressurizing pulses corresponding to the number of times that the pump is stroked, and the bursts are separated by time intervals during which the pump is not stroked. The invention of that patent application concerns self-compensation for changing pump efficiency as the pump ages.
The present invention concerns a leak test system and method that in a preferred embodiment employs a diaphragm pump that is stroked to force air into the space being tested. The pump is operated in a manner that creates a succession of pressurizing pulse bursts. Each burst contains a number of pressurizing pulses corresponding to the number of times that the pump is stroked, and the bursts are separated by time intervals during which the pump is not stroked. The present invention departs from the content of Ser. No. 09/896,247 in that it involves measuring leakage in a novel manner that can contribute to more consistent accuracy of results in less than perfectly stable ambient conditions for a leak test. This is because measurements can be taken in greater number and at greater frequency. Because of these larger numbers, any momentary irregularity or disturbance that affects a small percentage of the measurements as they are being taken may well have less effect on the final result than if one measurement of a fewer number of measurements were affected.
That said, the invention does not necessarily require the taking of multiple measurements, and in fact it is possible to perform an acceptable test using a single measurement taken at a certain point in the test, such as at the end of an allotted test time.
Another advantage of the invention is that it can be implemented in software that operates existing hardware in a new and different way according to the inventive principles.
One general aspect of the invention relates to a leak test system for a motor vehicle fuel system that holds volatile liquid fuel for operating the vehicle. The leak test system comprises a processor for establishing steps of a leak test and a pump for forcing air under pressure into vapor containment space of the fuel system during a leak test. The pump operates in accordance with steps established by the processor to create a superatmospheric pressure in the space during an initial step of the leak test. After completion of the initial step and in the presence of leakage from the space, a further step is performed. The pump operates according to a repeating duty cycle wherein the duty cycle comprises forcing air into the space in successive bursts of individual air pulses, each of which bursts is delayed from an immediately prior burst by a time interval during which pressure in the space decays because of the leakage. The processor processes, according to an algorithm, data corresponding to the duty cycle and uses the result of such processing to set the number of individual air pulses in each of subsequent bursts for regulating the pressure in the space substantially to the target pressure. Upon attainment of substantial stability in regulation of the pressure in the space at the target pressure, as indicated by attainment of substantial stability of the duty cycle, the processor processes data correlated with the duty cycle to indicate leakage from the space.
A further aspect of the invention relates to a leak test method for such a motor vehicle fuel system. The method comprises forcing air under pressure into vapor containment space of the fuel system during a leak test in accordance with steps of the method. During an initial step of the method, the forcing of air into the space creates in the space a superatmospheric pressure suitable for performing the leak test. After completion of the initial step and in the presence of leakage from the space, a pump is operated according to a repeating duty cycle wherein the duty cycle comprises forcing air into the space in successive bursts of individual air pulses, each of which bursts is delayed from an immediately prior burst by a time interval during which pressure in the space decays because of the leakage. Data corresponding to the duty cycle is processed according to an algorithm, and the result of such processing is used to set the number of individual air pulses in each of subsequent bursts for regulating the pressure in the space substantially to the target pressure. Upon attainment of substantial stability in regulation of the pressure in the space at the target pressure, as indicated by attainment of substantial stability of the duty cycle, data correlated with the duty cycle is processed to indicate leakage from the space.